surfaceflinger: discard stale commands

When we skip validate, we may have stale commands on the command
buffer.  They should be discarded.

Bug: 62906801
Test: manual
Change-Id: Ie7921ba75ab2252b834cda6924e1fcfdf236f759
(cherry picked from commit 0c6ce4608fc022608537162dd919e7f729b9cb77)
3 files changed